Chainsaws mounted on tree harvester vehicles usually comprise a saw unit mounted on an articulated arm which can be extended to the tree to be felled. The same saw unit can then also be used for bucking the tree to desired lengths. The saw unit comprises the chainsaw with a saw chain, a guide bar and a motor, as well as gripping arms and feeder wheels to hold and position the tree trunk.
The guide bar is attached to the motor by clamping its clamping end between two clamping blocks. The saw chain runs around the guide bar guided by a groove, and the guide bar edges on each side of the groove usually have at least partially hardened surfaces (i.e., surfaces which are harder than the body of the guide bar) for smooth sliding of the chain for an extended time period.
Tree harvester guide bars are subjected to very great stresses in service, especially if the support of the vehicle against the ground fails, or the gripper arms of the saw unit slip on the tree trunk, with the result that the weight of the tree is transferred to the guide bar. The largest stresses occur immediately in front of the clamping blocks. The guide bar will often be permanently bent in this region when overloaded, and it has always been a desire that bent guide bars should be straightenable for further use. Three obstacles to this goal have been cracking of hardened sliding surfaces, fractures in the welds joining the plates in laminated guide bars, and abrupt local buckling of a guide bar edge into the groove, making it too narrow for the saw chain to pass easily, if at all, therethrough.
Many guide bar designs have been suggested to avoid or reduce these obstacles. Cracks in the sliding surfaces are minimized if the hardening is done only in the regions most subject to wear, as in U.S. Pat. No. 2,962,812, or specifically omitted in the critical region in front of the clamping region as in U.S. Pat. No. 2,897,856. The neighborhood of the oil supply holes within the clamping region covered by the clamping blocks in operation are also preferably unhardened.
Fracture of welds are purported to be minimized if spot welds are made especially close and numerous within the critical region and afterwards annealed as in U.S. Pat. No. 5,052,109, or if the welding is done before total hardening and edge hardening as in SE Patent 469,324. Local buckling is a major disadvantage with guide bars according to U.S. Pat. No. 5,052,109 and will also occur with bars according to SE Patent 469,324, although at a much higher load.